The present invention relates to a method and apparatus for alternating the pressure points of a low air loss bed. The advantages of such an apparatus as well as the particular problems solved by this invention are discussed below.
Low air loss beds use inflatable cushions or air bags as the supporting surface for a patient. By using a fluid supporting medium such as air within the bags, an irregularly shaped body placed on top of the air bags will deform the supporting surface in such a manner so as to provide a more uniform distribution of load bearing pressure points than can be attained with a conventional mattress. When a patient lies supinely on a flat surface, or even on a conventional mattress, most of the load is borne by protuberances of the posterior surface of the body such as the heels, the buttocks, the scapula, and the occipital region of the head. The relatively small areas of soft tissue at these points are then subjected to high pressures by being compressed between the skeleton and the supporting surface. When this pressure becomes great enough to cause collapse of small capillaries and veins, pressure sores may result. By uniformly distributing the supporting pressure points along the body surface, the pressure at these critical areas can be reduced. Patients are also predisposed to pressure sores by the accumulation of moisture at the skin surface. For this reason, air bags which are permeable to water vapor are preferred. A continuous flow of air through the bags from a source of pressurized air is then necessary to remove the water vapor, the air being exhausted through separate outlets or pores in the fabric of the bags. It is this feature which distinguishes a low air loss bed from a simple inflatable mattress.
U.S. Pat. No. 3,822,425 discloses a low air loss bed consisting of a number of cells or bags, each having a surface which supports the patient, formed from a material which is said to be gas permeable but non-permeable to liquids and solids. That patent also discloses an air supply for inflating the cells to the required pressure and outlets or exhaust ports in the cells to allow the escape of air. The bed disclosed is divided into sections, each of which comprises a group of air bags. Each section is provided with a pressure sensor and a control valve allowing each section of the bed to be inflated to different pressures. Alternatively, the air pressure in each section is controlled by valves in the outlets from the section.
Low air loss beds of the type disclosed in the '425 patent are typically also provided with means for adjusting the patient's attitude on the bed. For instance, the head of the bed can be raised to sit the patient up or the angle of the entire frame of the bed can be changed with respect to the horizontal when, for therapeutic reasons, the patient is placed in the Trendelenburg or reverse Trendelenburg positions. Those changes require re-adjustment of the air supply in each section of air bags. Movement of the patient may also necessitate adjustment of the pressure in each section as the patient's weight distribution on the bed changes.
Various other approaches have been taken to solving the problem of preventing bedsores in bedridden patients. One common approach is the use of what is referred to as an alternating pressure mattress. Such mattresses are comprised of two sets of alternately inflatable, interdigitated cells or tubes either connected to form a mattress or formed from closely approximated sheets of air impermeable material which have been heat sealed or otherwise bonded at the edges and with tubes or channels formed therein to form alternating cells. Such mattresses are disclosed in, for example, U.S. Pat. Nos. 3,595,223, 4,193,149, and 4,391,009. In all such mattresses, a separately controllable air supply is provided to each set of cells. By alternately inflating and deflating each set of cells in opposite phase to the other set, the supporting surface of the mattress is alternated between each set of cells. The object of these devices is to periodically relieve and transfer points of contact between the patient's body and the supporting surface. These devices, however, are not low air loss beds. This means that the pressure in each set of cells is merely varied from a full inflate to a full deflate condition. Alternating pressure mattresses have not been designed in the past to provide the uniform patient support provided by low air loss beds. Some have even been designed to do the opposite in order to provide a vigorous massaging action.
The degree of uniformity of support provided by a low air loss bed varies with the pressure existing within the air bags for any given patient. The pressure exerted against a body resting on an air bag is approximately equal to the air pressure within the bag when the air bag is deformed only to an extent which flattens the body contacting surface of the bag. Further deformation increases the pressure exerted by the bag surface against the body because the body contacting surface of the bag, in addition to being pushed by the air pressure within the bag, is pulled by the tension existing in the bag fabric surrounding the body. This tension is maintained by the air pressure exerted against the inner surfaces of the bag which surround the body. In any case, of course, the pressure exerted against the body by the bag surface integrated over the total body contacting surface equals the weight of the body.
In order to maximize the uniformity of support provided by a low air loss bed, the air pressure within the bags should be maintained at a value low enough to allow the supporting surface to be deformed in order to increase the weight bearing surface area but not low enough that too much tension is produced in the bag fabric surrounding the body contacting surface. Such tension in the fabric interferes with the deformation of the supporting surface by protruding body parts. Therefore, for a body of any particular size and weight, there exists a pressure value which maximizes the degree of uniformity of support provided to the body by the air bag. Since weight is not distributed evenly on the human frame, this ideal pressure value varies with different body regions. Heavier regions such as the buttocks require greater pressure to achieve uniform support while lighter regions such as the feet require less pressure.
Of course, no matter how uniform the support provided to a patient by a low air loss bed, areas of the patient's body necessarily are subjected to some pressure. Furthermore, for the reasons discussed above, protruding areas of the body are subjected to relatively greater pressure. It would be advantageous, therefore, for the pressure points in a low air loss bed to be periodically shifted from one body area to another without compromising the uniform supporting characteristics of a low air loss bed.
A low air loss bed which also incorporates some of the characteristics of an alternating pressure mattress would present a number of advantages. Periodically relieving alternate body areas of pressure would ensure that no body area becomes completely ischemic due to excessive support pressure. Also, if the bags are positioned transversely, periodically increasing the pressure to alternate body areas has the effect of compressing subcutaneous veins which, owing to the one-way valves existing in human veins, provides an impetus to the flow of blood back to the heart. Not only does this improve arterial circulation, but it also makes less likely venous pooling which can cause edema and predispose the patient to pressure sores.
In order not to compromise the uniform supporting characteristics of the low air loss bed, however, the air bags cannot simply be separated into two sets, interdigitated, connected to two separate pressure sources, and then alternately inflated and deflated. As aforesaid, an ideal air bag pressure exists for each patient of a particular size and weight which maximizes the degree of uniformity of support. After being determined empirically, the ideal pressure should be maintained in each bag within limits. Furthermore, this ideal pressure varies with the particular body region being supported by a group of air bags. What is needed, therefore, is a low air loss bed which allows operator selection of the air bag pressure for each set of air bags supporting a particular body region and maintains that ideal Pressure as a setpoint or baseline value about which the pressures are raised and lowered as the pressure points are alternately shifted from one set of interdigitated bags to another.
It would be further advantageous for such a low air loss bed to allow the operator to select the degree of relative increase and decrease from the ideal pressure for each set of air bags when in the alternating pressure mode.
Yet another advantage would accrue if the setpoint pressure could be automatically changed for the different sets of air bags as different sections of the bed frame are adjusted from the horizontal.